1. Field of the Invention
The present invention relates to apparatus for monitoring the operation of a battery generally and, more particularly, but not by way of limitation, to novel method and means for adjusting a battery monitor based on the rate of current drawn from the battery.
2. Background Art
In U.S. Pat. No. 4,560,937, issued Dec. 24, 1985, to Eugene P. Finger, and titled BATTERY STATE OF CHARGE METERING METHOD AND APPARATUS, incorporated by reference hereinto, there is disclosed a metering system for measuring and indicating the state of charge of an electric storage battery which includes a digital integrator having a plurality of digital states operable to store a numerical value in binary digital form which is indicative of the state of charge, the integrator being operable to change the numerical value stored therein in one direction in response to detected decreases in state of charge as the battery is being discharged. The integrator generates a series of voltage pulses on a single output line, each pulse corresponding in length to the binary digital value stored in a predetermined number of the highest order binary stages of the digital integrator to provide a time-based resultant indication of the binary digital value. A filter circuit is provided for filtering the pulses to provide a substantially smooth analog output voltage signal having a voltage amplitude corresponding to the summation of the pulses.
The system of the above-referenced patent, including, in detail, aspects thereof particularly relevant to the present disclosure, is shown schematically on FIG. 1. For convenient reference to the same-numbered figure of that patent, the same reference numerals have been used, except that, in the present FIG. 1, those reference numerals are preceded by the numeral "6". The present invention is applicable to other battery monitoring systems, as will be described later, but the operation of the system of the above-referenced patent will be set forth in some detail for background on the operation of such systems generally.
The metering system is connected to an electric storage battery 610 and to a load 616. A fixed fraction of the battery voltage is supplied from a voltage divider consisting of resistors 626 and 628 through a connection 632 to a threshold comparator 634. The fixed fraction of the battery voltage is compared by comparator 634 with a reference voltage supplied through a connection 636 from a reference voltage slope network 638 which is described more fully below. Whenever load is applied to battery 610, the resultant downward excursion in the battery terminal voltage is detected by comparator 634. If the downward excursion is below a threshold, as determined by the reference voltage on connection 636, a resultant signal is provided to a digital circuit unit 642 at "IN" through a resistor 640. Digital circuit unit 642 generates a continuous series of voltage pulses at output terminal "FB" to output connection 646, the voltage pulses respectively corresponding in length to the binary digital value stored in the digital circuit unit. The pulses are then filtered in a low pass filter combination consisting of a resistor 648 and a capacitor 650 to provide a substantially smooth analog output voltage signal having a voltage amplitude corresponding to the binary digital value stored in digital circuit unit 642. That voltage is amplified by an operational amplifier 652 connected as a voltage follower amplifier. The resultant amplified output voltage optionally may be supplied through a connection 654 to a voltmeter 656 to thereby visually indicate the state of charge of battery 610.
A resistor 6131 and a capacitor 6132 connected to a clock oscillator (not shown) within digital circuit unit 642 determine the frequency of operation of the unit and, therefore, the integration rate thereof.
The output from amplifier 652 is also supplied to reference voltage slope network 638 so that the network may generate a variable reference signal which is a function of the analog output voltage signal. The reference voltage is also determined in part by a substantially constant reference voltage signal on input connection 658, the constant reference voltage signal being derived from other portions of the system (not shown).
It is apparent from an inspection of network 638 that, in the absence of an input to the network from amplifier 652, the reference voltage output on connection 636 which sets the voltage threshold for comparator 634 would be a constant fraction of the substantially constant reference voltage on input connection 658, as determined by voltage divider resistors 682 and 684. However, the input from amplifier 652 is operable to reduce the threshold voltage level at connection 636 as the discharge of battery 610 progresses, as recorded by the integrator (not shown) contained within digital circuit unit 642. This is appropriate, since the terminal voltage of battery 610 will decrease for each load current level as the discharge of the battery progresses. Thus, the downward adjustment of the reference voltage threshold as a function of the battery discharge condition prevents faster integration than is warranted. The variable reference voltage may approximate a nominal loaded open circuit battery terminal voltage at the various states of discharge of the battery. The voltage supplied through amplifier 652 to network 638 may be characterized as a feedback voltage.
The voltage output from amplifier 652 supplied to network 638 is at its highest when battery 610 is fully charged and when the system is registering a fully charged condition. However, the voltage relationships are such that the voltage at the circuit node between resistors 674 and 676 is always somewhat below the voltage at the node between the vertical leg resistors 682 and 684 so that the cross connection formed by resistors 678 and 680 causes a downward translation of the voltage at the node between resistors 682 and 684 which is the output voltage reference on connection 636. Adjustment of variable resistor 678 changes this downward offset, with reducing the value of the variable resistor increasing the offset, and increasing the value of the variable resistor reducing the offset. As the system indicates a depletion of the charge, on the basis of the number stored in the integrator (not shown) contained within digital circuit unit 642, the voltage output level of amplifier 652 is reduced and the reference voltage is correspondingly reduced, producing a sloping characteristic, or transform. This threshold reference voltage characteristic is illustrated on FIG. 2 as curve 686.
Curve 686 on FIG. 2 represents a plot of the threshold voltage output from network 638 on connection 636 versus the state of charge, as indicated by the numerical value stored in digital circuit unit 642. It will be seen that the voltage value slopes downwardly as the percent of charge remaining decreases, since the battery terminal voltage generally decreases as the charge is depleted and, therefore, the resulting lower voltage excursions on load look like heavier loads to the metering system.
Curves 690 and 692 on FIG. 2 illustrate the performance of network 638 with greater vertical displacements caused by successive reductions in the variable resistor 678. It is a very useful feature of network 638 that adjustments in the vertical displacement of the threshold voltage characteristic also provide an appropriate change in the slope of the characteristic. The main reason for providing the adjustments available from variable resistor 678 is to accommodate for different objectives of different users in terms of battery life versus maximum battery energy output per charge. For example, for a user who places the major emphasis on battery life, the battery must be considered as fully discharged at a higher specific gravity per cell, and at a higher final battery terminal voltage than would otherwise be attained. For that user, network 638 might be adjusted to provide for the voltage threshold curve 686. For another example, for a user who wishes to obtain more energy from the battery by discharging the battery more deeply, even though battery life may be compromised, one or the other of curves 690 and 692 might be selected by suitable adjustment of variable resistor 678.
It has been found that a "factory set" variable resistor 678 is quite satisfactory in most applications, where the factory setting takes into account anticipated usage of the battery. In other cases, however, setting of variable resistor 678 must be done in the field to account for unknown variables. The setting of variable resistor 678 is a compromise in a single, simple adjustment to accommodate such variables as battery type, battery size, desired depth-of-discharge, and average loading.
It has been found, further, that the system of FIG. 1, whether variable resistor 678 is factory or field set, is quite satisfactory in applications in which batteries are used in a day's work with a "normal" statistical mix of loads and with a properly sized battery. However, providing variable resistor 678 with a "fixed" setting may not be satisfactory when the types of use a battery encounters vary considerable from day to day. An example of such variable use is in a paper mill where, on one day, a battery-operated forklift truck might be used to relocate pallet loads of facial tissue, resulting in only light battery use and, on another day, the same forklift truck might be used to carry logs, requiring heavy battery use. In such cases of wide variability of use, it would be desirable to be able to automatically alter threshold voltage depending on the type of use of the battery. Extreme variations in use can also be compensated for by altering the integration rate, either with or without a change in threshold voltage.
Accordingly, it is a principal object of the present invention to provide method and means for automatically adjusting a battery operation monitor to compensate for variations in use of the battery.
It is a further object of the invention to make such adjustments by reference to the rate of discharge of the battery.
It is another object of the invention to provide such means that can be economically constructed.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.